3He thermal neutron detectors and electronics are the backbone of safeguards neutron measurements and have performed relatively well in the past three decades without significant development because conventional 3He detectors have important beneficial properties, such as a very high cross-section for thermal neutrons and a plateau of the counting characteristics due to separated neutron and gamma distributions. The plateau mitigates the tube and electronics gain instability, and thus enables high precision measurements with relatively simple readout electronics. Conventional 3He counters have a gas pressure between 4 and 10 atmospheres (atm), a tube diameter of one inch, and an anode wire of 0.002 inches. The most common electronics sets consist of bipolar shapers with short time constants followed by a discrimination and logic pulse driver. Three types of electronics packages produced by PDT™ (as a standalone device), Bot Engineering™, and Canberra™ (JAB-01 boards based on the Amptek™ A-111 hybrid chip) are the preferred choice for conventional safeguards measurements.
The main limitations of 3He technology using standard electronics are dead time and gamma sensitivity for more demanding applications, such as multiplicity counting and spent fuel measurements. While both dead time and gamma pile-up would benefit from shorter shaping time, when the shaping time constant becomes too short, multiple triggering (i.e., so called double pulsing) occurs due to variations in the rise time of detector current pulses.
Some applications call for the use of tubes with high pressure and slow gas admixture, which makes the detector pulse slower. In this case, low dead time electronics with short shaping time will not be able to collect sufficient charge, resulting in low sensitivity. One approach that was tried was to use thinner anode wire that provided higher detector gain. However, this slowed down the detector pulse even more, and made the detector vulnerable to space charge effects. This approach would not be suitable for new safeguards challenges, such as direct measurement of plutonium (Pu) in spent fuel for the Next Generation Safeguards Initiative (NGSI), which requires the detectors to be placed in close proximity to a spent fuel assembly. Subjecting the detectors to high gamma and neutron fluxes can lead to space charge effects, requiring the use of slow but radiation tolerant gas admixes, and yielding neutron count rates many times higher than the counting capabilities of conventional detectors and electronics.
Conventional approaches to mitigate these high rate effects include tweaking the time constants of the electronics and experimental ad hoc gas mixtures and pressure that speed up the detector response. Unfortunately, these approaches have a limited effect because, while the detector dead time at a given pressure and tube geometry is reduced with increased admix concentration, the gamma sensitivity is increased since more energy can be deposited in the gas (e.g., fast proportional gas mixtures, such as Ar+CH4, Ar+CO2, or modern CF4). The operation in high gamma fields is additionally aggravated due to the limiting situation of self-shielding effects around the anode (i.e., space charge) causing a gain shift, and thus a change of detection efficiency.
The gamma radiation also deposits energy in the thermal neutron detectors. While the gamma response of neutron detector is considered to be unwanted and parasitic, the simultaneous readout of gamma and neutron signals from same detector is a desirable feature that can save cost, space, and measurement time.
The currently used Direct Current (DC) mode gamma ionization chambers have a nonlinear response to the measured gross gamma radiation. The currently used front end electronics converting the ionization chamber DC current into easy to transmit count logic pulses with a frequency proportional to the detector current have limited dynamic range and accuracy. The recent trend for higher accuracy and extended dynamic range of these measurements requires improved gross gamma readout for these detectors.
Additionally, increasing the 3He gas pressure does not produce a commensurate increase in detection efficiency due to the self-shielding effects for thermal neutrons. The net result is an increased detector cost-to-efficiency ratio and gamma sensitivity as there is no self-shielding for gammas. For systems requiring a large amount of (currently very expensive) 3He, the diminishing efficiency return is important.
In the light of the massive research and development (R&D) effort motivated by the 3He gas shortage, the improvement in 3He detectors themselves for safeguards was neglected. Most of these mitigation approaches have successfully addressed specific problems and applications for moderate count rates. However, many interesting applications involve significant count rate increases and require a comprehensive detector and electronics redesign approach.
Thus, a technology gap exists between standard 3He detectors or alternatives and what is required to provide high count rate detectors. Accordingly, improved thermal neutron counters and electronics may be beneficial.